数据结构(集合)学习之Map（二）

集合

框架关系图

补充：HashTable父类是Dictionary，不是AbstractMap。

一：HashMap中的链循环：

一般来说HashMap中的链循环会发生在多线程操作时（虽然HashMap就不是线程安全的，一般多线程的时候也不会用它，但这种情况难免会发生）

以JDK1.7为例：

    void addEntry(int hash, K key, V value, int bucketIndex) {
        if ((size >= threshold) && (null != table[bucketIndex])) {
            resize(2 * table.length);//扩容方法，参数为原来数组长度的2倍
            hash = (null != key) ? hash(key) : 0;
            bucketIndex = indexFor(hash, table.length);
        }
 
        createEntry(hash, key, value, bucketIndex);
    }
    void resize(int newCapacity) {
        Entry[] oldTable = table;//数组原来的数据
        int oldCapacity = oldTable.length;//原来数组的长度
        if (oldCapacity == MAXIMUM_CAPACITY) {
            threshold = Integer.MAX_VALUE;
            return;
        }
 
        Entry[] newTable = new Entry[newCapacity];//以新的初始容量创建新的数组
        transfer(newTable, initHashSeedAsNeeded(newCapacity));//转换方法
        table = newTable;
        threshold = (int)Math.min(newCapacity * loadFactor, MAXIMUM_CAPACITY + 1);
    }
    void transfer(Entry[] newTable, boolean rehash) {
        int newCapacity = newTable.length;
        for (Entry<K,V> e : table) {
            while(null != e) {
                Entry<K,V> next = e.next;
                if (rehash) {
                    e.hash = null == e.key ? 0 : hash(e.key);
                }
                int i = indexFor(e.hash, newCapacity);//e是oldTable上的Entry对象，这里算出新数组的下标
                e.next = newTable[i];
                newTable[i] = e;
                e = next;
            }
        }
    }

通过代码分析：当old Table中的数值都转移到newTable时，元素的顺序会进行颠倒。效果图如下：

在这种情况下，假设有两个线程同时进行扩容的操作且都进入了transfer方法时，但是因为某个情况让线程一先执行完，线程二才开始执行。

这个时候如果线程一执行完了，线程二开始的状态就是这样：

这个时候：线程二的e还是A对象，e.Next（A.Next）还是指向B，但是因为线程一的关系，实际上此时B.Next已经指向了A。于是就产生了循环：A.Next指向B，B.Next指向A。

二：hashtable

HashTable是Map接口下的一个子类，线程安全的，1.7和1.8差别不大，常和HashMap作比较，部分源码如下：

 //hashTable默认初始容量和加载因子
     public Hashtable() {
         this(11, 0.75f);
     }
 
 //Put方法
     public synchronized V put(K key, V value) {
         // 不允许存null
         if (value == null) {
             throw new NullPointerException();
         }
 
         // key相同时新值代替老值，并把老值返回出去
         Entry tab[] = table;
         int hash = hash(key);//哈希算法
         int index = (hash & 0x7FFFFFFF) % tab.length;
         for (Entry<K,V> e = tab[index] ; e != null ; e = e.next) {
             if ((e.hash == hash) && e.key.equals(key)) {
                 V old = e.value;
                 e.value = value;
                 return old;
             }
         }
 
         modCount++;
         if (count >= threshold) {
             rehash();//扩容
 
             tab = table;
             hash = hash(key);
             index = (hash & 0x7FFFFFFF) % tab.length;
         }
 
         Entry<K,V> e = tab[index];
         tab[index] = new Entry<>(hash, key, value, e);
         count++;
         return null;
     }
 //哈希算法
     private int hash(Object k) {
         return hashSeed ^ k.hashCode();
     }
 //扩容方法
     protected void rehash() {
         int oldCapacity = table.length;
         Entry<K,V>[] oldMap = table;
 
         int newCapacity = (oldCapacity << 1) + 1;//扩容后容量为原来的2倍再加1
         if (newCapacity - MAX_ARRAY_SIZE > 0) {
             if (oldCapacity == MAX_ARRAY_SIZE)
                 // Keep running with MAX_ARRAY_SIZE buckets
                 return;
             newCapacity = MAX_ARRAY_SIZE;
         }
         Entry<K,V>[] newMap = new Entry[newCapacity];
 
         modCount++;
         threshold = (int)Math.min(newCapacity * loadFactor, MAX_ARRAY_SIZE + 1);
         boolean rehash = initHashSeedAsNeeded(newCapacity);
 
         table = newMap;
 
         for (int i = oldCapacity ; i-- > 0 ;) {
             for (Entry<K,V> old = oldMap[i] ; old != null ; ) {
                 Entry<K,V> e = old;
                 old = old.next;
 
                 if (rehash) {
                     e.hash = hash(e.key);
                 }
                 int index = (e.hash & 0x7FFFFFFF) % newCapacity;
                 e.next = newMap[index];
                 newMap[index] = e;
             }
         }
     }
 //remove方法
     public synchronized V remove(Object key) {
         Entry tab[] = table;
         int hash = hash(key);
         int index = (hash & 0x7FFFFFFF) % tab.length;
         for (Entry<K,V> e = tab[index], prev = null ; e != null ; prev = e, e = e.next) {
             if ((e.hash == hash) && e.key.equals(key)) {
                 modCount++;
                 if (prev != null) {
                     prev.next = e.next;
                 } else {
                     tab[index] = e.next;
                 }
                 count--;
                 V oldValue = e.value;
                 e.value = null;
                 return oldValue;
             }
         }
         return null;
     }
 //get方法
     public synchronized V get(Object key) {
         Entry tab[] = table;
         int hash = hash(key);
         int index = (hash & 0x7FFFFFFF) % tab.length;
         for (Entry<K,V> e = tab[index] ; e != null ; e = e.next) {
             if ((e.hash == hash) && e.key.equals(key)) {
                 return e.value;
             }
         }
         return null;
     }
 //keySet方法
     public Set<K> keySet() {
         if (keySet == null)
             keySet = Collections.synchronizedSet(new KeySet(), this);
         return keySet;
     }
 
     private class KeySet extends AbstractSet<K> {
         public Iterator<K> iterator() {
             return getIterator(KEYS);
         }
         public int size() {
             return count;
         }
         public boolean contains(Object o) {
             return containsKey(o);
         }
         public boolean remove(Object o) {
             return Hashtable.this.remove(o) != null;
         }
         public void clear() {
             Hashtable.this.clear();
         }
     }
 //entrySet方法
     public Set<Map.Entry<K,V>> entrySet() {
         if (entrySet==null)
             entrySet = Collections.synchronizedSet(new EntrySet(), this);
         return entrySet;
     }
 
     private class EntrySet extends AbstractSet<Map.Entry<K,V>> {
         public Iterator<Map.Entry<K,V>> iterator() {
             return getIterator(ENTRIES);
         }
 
         public boolean add(Map.Entry<K,V> o) {
             return super.add(o);
         }
 
         public boolean contains(Object o) {
             if (!(o instanceof Map.Entry))
                 return false;
             Map.Entry entry = (Map.Entry)o;
             Object key = entry.getKey();
             Entry[] tab = table;
             int hash = hash(key);
             int index = (hash & 0x7FFFFFFF) % tab.length;
 
             for (Entry e = tab[index]; e != null; e = e.next)
                 if (e.hash==hash && e.equals(entry))
                     return true;
             return false;
         }
 //clear方法
     public synchronized void clear() {
         Entry tab[] = table;
         modCount++;
         for (int index = tab.length; --index >= 0; )
             tab[index] = null;
         count = 0;
     }
 //size方法和isEmpty方法
     public synchronized int size() {
         return count;
     }
 
     public synchronized boolean isEmpty() {
         return count == 0;
     }

可以看出和HashMap比较，有以下特点：

1、初始容量为11。

2、扩容方法为：oldTable.Length*2+1。

3、不允许存null值。

4、计算hash，和获取数组下标不同：

HashTable：

int hash = key.hashCode();
int index = (hash & 0x7FFFFFFF) % tab.length;

HashMap：

int hash = hash(key);//
int i = indexFor(hash, table.length);

5、HashTable线程安全，主要的方法都用了synchronized，加锁。

6、HashMap和HashTable父类不同

public class HashMap<K,V> extends AbstractMap<K,V> implements Map<K,V>, Cloneable, Serializable
public class Hashtable<K,V> extends Dictionary<K,V> implements Map<K,V>, Cloneable, java.io.Serializable

7、HashTable线程安全，但因为每个主要方法上都加上了锁，所以在执行效率上会慢很多。

三：其他线程安全Map

1、synchronizedMap

通过工具类Collections的方法，返回一个HashMap，例如：Map<String,String> map = Collections.synchronizedMap(new HashMap<String,String>());

     public static <K,V> Map<K,V> synchronizedMap(Map<K,V> m) {
         return new SynchronizedMap<>(m);
     }
 
     private static class SynchronizedMap<K,V>
         implements Map<K,V>, Serializable {
         private static final long serialVersionUID = 1978198479659022715L;
 
         private final Map<K,V> m;     // Backing Map
         final Object      mutex;        // Object on which to synchronize
 
         SynchronizedMap(Map<K,V> m) {
             if (m==null)
                 throw new NullPointerException();
             this.m = m;
             mutex = this;
         }
 
         SynchronizedMap(Map<K,V> m, Object mutex) {
             this.m = m;
             this.mutex = mutex;
         }
 
         public int size() {
             synchronized (mutex) {return m.size();}
         }
         public boolean isEmpty() {
             synchronized (mutex) {return m.isEmpty();}
         }
         public boolean containsKey(Object key) {
             synchronized (mutex) {return m.containsKey(key);}
         }
         public boolean containsValue(Object value) {
             synchronized (mutex) {return m.containsValue(value);}
         }
         public V get(Object key) {
             synchronized (mutex) {return m.get(key);}
         }
 
         public V put(K key, V value) {
             synchronized (mutex) {return m.put(key, value);}
         }
         public V remove(Object key) {
             synchronized (mutex) {return m.remove(key);}
         }
         public void putAll(Map<? extends K, ? extends V> map) {
             synchronized (mutex) {m.putAll(map);}
         }
         public void clear() {
             synchronized (mutex) {m.clear();}
         }
 
         private transient Set<K> keySet = null;
         private transient Set<Map.Entry<K,V>> entrySet = null;
         private transient Collection<V> values = null;
 
         public Set<K> keySet() {
             synchronized (mutex) {
                 if (keySet==null)
                     keySet = new SynchronizedSet<>(m.keySet(), mutex);
                 return keySet;
             }
         }
 
         public Set<Map.Entry<K,V>> entrySet() {
             synchronized (mutex) {
                 if (entrySet==null)
                     entrySet = new SynchronizedSet<>(m.entrySet(), mutex);
                 return entrySet;
             }
         }
 
         public Collection<V> values() {
             synchronized (mutex) {
                 if (values==null)
                     values = new SynchronizedCollection<>(m.values(), mutex);
                 return values;
             }
         }
 
         public boolean equals(Object o) {
             if (this == o)
                 return true;
             synchronized (mutex) {return m.equals(o);}
         }
         public int hashCode() {
             synchronized (mutex) {return m.hashCode();}
         }
         public String toString() {
             synchronized (mutex) {return m.toString();}
         }
         private void writeObject(ObjectOutputStream s) throws IOException {
             synchronized (mutex) {s.defaultWriteObject();}
         }
     }

可以看出，原理就是在原来Map的基础上，加上锁synchronized (mutex)，来让线程变得安全，但和HashTable一样，效率慢。

2、ConcurrentHashMap

JDK1.7中的ConcurrentHashMap：

ConcurrentHashMap是线程安全的Map，继承AbstractMap。是和HashTable把整合数组共享一把锁不同，ConcurrentHashMap采用分段的思想，把HashMap分为多个段进行加锁的操作，这样既保证了线程安全性，又不会使效率像HashTable那样低。

ConcurrentHashMap的特点是分段式加锁，并利用Unsafe对象和volatile关键字来实现线程安全，他比较明显的一个局限性是并发级别一旦指定就不再更改。

结构：

代码：

 //默认初始容量（2的幂），实际上是指的HashEntry的数量
     static final int DEFAULT_INITIAL_CAPACITY = 16;
 //默认加载因子
     static final float DEFAULT_LOAD_FACTOR = 0.75f;
 //默认并发级别（实际上指的Segmengt对象的数量，初始化后不可更改）
     static final int DEFAULT_CONCURRENCY_LEVEL = 16;
 //最大容量小于等于2的30次幂
     static final int MAXIMUM_CAPACITY = 1 << 30;
 //最小SEGMENT容量(一个Segment里至少有两个HashEntry)
     static final int MIN_SEGMENT_TABLE_CAPACITY = 2;
 //最大SEGMENT容量容量
     static final int MAX_SEGMENTS = 1 << 16;
 //加锁之前的重试次数
     static final int RETRIES_BEFORE_LOCK = 2;
 //Segment数组
     final Segment<K,V>[] segments;
 //计算segment位置的掩码，用于计算Segment[]下标
     final int segmentMask;
 //用于算segment位置时,hash参与运算的位数
     final int segmentShift;
 //内部类HashEntry，相当于HashMap中Entry[]中的Entry对象
     static final class HashEntry<K,V> {
         final int hash;
         final K key;
         volatile V value;
         volatile HashEntry<K,V> next;
 
         HashEntry(int hash, K key, V value, HashEntry<K,V> next) {
             this.hash = hash;
             this.key = key;
             this.value = value;
             this.next = next;
         }
 
         final void setNext(HashEntry<K,V> n) {
             UNSAFE.putOrderedObject(this, nextOffset, n);
         }
 
         static final sun.misc.Unsafe UNSAFE;
         static final long nextOffset;
         static {
             try {
                 UNSAFE = sun.misc.Unsafe.getUnsafe();
                 Class k = HashEntry.class;
                 nextOffset = UNSAFE.objectFieldOffset
                     (k.getDeclaredField("next"));
             } catch (Exception e) {
                 throw new Error(e);
             }
         }
     }
 //hash函数
     private int hash(Object k) {
         int h = hashSeed;
 
         if ((0 != h) && (k instanceof String)) {
             return sun.misc.Hashing.stringHash32((String) k);
         }
 
         h ^= k.hashCode();
 
         h += (h <<  15) ^ 0xffffcd7d;
         h ^= (h >>> 10);
         h += (h <<   3);
         h ^= (h >>>  6);
         h += (h <<   2) + (h << 14);
         return h ^ (h >>> 16);
     }
 //ConcurrentHashMap的构造方法
     public ConcurrentHashMap(int initialCapacity, float loadFactor, int concurrencyLevel) {//初始容量，加载因子，并发级别
         if (!(loadFactor > 0) || initialCapacity < 0 || concurrencyLevel <= 0)//保证参数不能小于0
             throw new IllegalArgumentException();
         if (concurrencyLevel > MAX_SEGMENTS)//保证并发级别不能大于最大Segment容量
             concurrencyLevel = MAX_SEGMENTS;
 
         int sshift = 0;
         int ssize = 1;
         while (ssize < concurrencyLevel) {
             ++sshift;//统计ssize左移的次数
             ssize <<= 1;//初始是1，通过左移然后2、4、8、16...作用是找到大于等于并发级别的2的“sshift”次幂
         }
         this.segmentShift = 32 - sshift;//用于Put方法中，求Segment[]数组的下标时用
         this.segmentMask = ssize - 1;//Segmengt[].length-1
         if (initialCapacity > MAXIMUM_CAPACITY)//如果初始容量大于最大
             initialCapacity = MAXIMUM_CAPACITY;//则选择最大容量作为容量
         int c = initialCapacity / ssize;//HashEntry[].length/Segmengt[].length，用来标记HashEntry的数量
         if (c * ssize < initialCapacity)//例如initialCapacity = 9，concurrencyLevel（ssize） = 8 ，则initialCapacity / ssize = 1，c * ssize < initialCapacity
             ++c; // 然后让C = 2, 此时 c * ssize >= initialCapacity  ，这里的目的就是保证数据都被Segmengt存储。
         int cap = MIN_SEGMENT_TABLE_CAPACITY;
         while (cap < c)//拿最小的容量和刚刚计算的 c 进行比较
             cap <<= 1; // 保证最小容量是2的n次幂
         Segment<K,V> s0 =
             new Segment<K,V>(loadFactor, (int)(cap * loadFactor),
                              (HashEntry<K,V>[])new HashEntry[cap]);//初始化Segment对象
         Segment<K,V>[] ss = (Segment<K,V>[])new Segment[ssize];//初始化Segment数组
         UNSAFE.putOrderedObject(ss, SBASE, s0); //调用Unsafe中的方法，作用是把刚刚初始化的Segemengt对象s0，放到刚刚初始化Segment[]数组中的Segment[0],也就是第一位
         this.segments = ss;
     }
 //Put方法
     public V put(K key, V value) {
         Segment<K,V> s;
         if (value == null)
             throw new NullPointerException();//不能传null，此处个人认为应该加个 key也不能为 null的判断
         int hash = hash(key);//计算hash值
         int j = (hash >>> segmentShift) & segmentMask; // 计算Segmengt[]下标，即数据存储的位置
         if ((s = (Segment<K,V>)UNSAFE.getObject(segments, (j << SSHIFT) + SBASE)) == null) //  调用Unsafe的方法，作用是获取Segmengt[j]的对象，如果等于空
             s = ensureSegment(j);//判断第j个位置是不是为空，保证线程安全性
         return s.put(key, hash, value, false);//调用Segmengt对象的put方法
     }
 
 //Segmengt的Put方法
         final V put(K key, int hash, V value, boolean onlyIfAbsent) {
             HashEntry<K,V> node = tryLock() ? null : scanAndLockForPut(key, hash, value);//加锁，保证线程安全
             V oldValue;
             try {
                 HashEntry<K,V>[] tab = table;
                 int index = (tab.length - 1) & hash;
                 HashEntry<K,V> first = entryAt(tab, index);//获取该位置的第一个元素，然后遍历链表
                 for (HashEntry<K,V> e = first;;) {
                     if (e != null) {
                         K k;
                         if ((k = e.key) == key ||
                             (e.hash == hash && key.equals(k))) {
                             oldValue = e.value;
                             if (!onlyIfAbsent) {
                                 e.value = value;
                                 ++modCount;
                             }
                             break;
                         }
                         e = e.next;
                     }
                     else {
                         if (node != null)
                             node.setNext(first);
                         else
                             node = new HashEntry<K,V>(hash, key, value, first);
                         int c = count + 1;
                         if (c > threshold && tab.length < MAXIMUM_CAPACITY)
                             rehash(node);
                         else
                             setEntryAt(tab, index, node);
                         ++modCount;
                         count = c;
                         oldValue = null;
                         break;
                     }
                 }
             } finally {
                 unlock();
             }
             return oldValue;
         }
 
         @SuppressWarnings("unchecked")
         private void rehash(HashEntry<K,V> node) {//扩容方法，只扩容HashEntry[]，没扩容Segmengt[]
 
             HashEntry<K,V>[] oldTable = table;
             int oldCapacity = oldTable.length;
             int newCapacity = oldCapacity << 1;
             threshold = (int)(newCapacity * loadFactor);
             HashEntry<K,V>[] newTable =
                 (HashEntry<K,V>[]) new HashEntry[newCapacity];
             int sizeMask = newCapacity - 1;
             for (int i = 0; i < oldCapacity ; i++) {
                 HashEntry<K,V> e = oldTable[i];
                 if (e != null) {
                     HashEntry<K,V> next = e.next;
                     int idx = e.hash & sizeMask;
                     if (next == null)
                         newTable[idx] = e;
                     else {
                         HashEntry<K,V> lastRun = e;
                         int lastIdx = idx;
                         for (HashEntry<K,V> last = next;
                              last != null;
                              last = last.next) {
                             int k = last.hash & sizeMask;
                             if (k != lastIdx) {
                                 lastIdx = k;
                                 lastRun = last;
                             }
                         }
                         newTable[lastIdx] = lastRun;
 
                         for (HashEntry<K,V> p = e; p != lastRun; p = p.next) {
                             V v = p.value;
                             int h = p.hash;
                             int k = h & sizeMask;
                             HashEntry<K,V> n = newTable[k];
                             newTable[k] = new HashEntry<K,V>(h, p.key, v, n);
                         }
                     }
                 }
             }
             int nodeIndex = node.hash & sizeMask;
             node.setNext(newTable[nodeIndex]);
             newTable[nodeIndex] = node;
             table = newTable;
         }
 
         private HashEntry<K,V> scanAndLockForPut(K key, int hash, V value) {
             HashEntry<K,V> first = entryForHash(this, hash);
             HashEntry<K,V> e = first;
             HashEntry<K,V> node = null;
             int retries = -1;
             while (!tryLock()) {
                 HashEntry<K,V> f;
                 if (retries < 0) {
                     if (e == null) {
                         if (node == null)
                             node = new HashEntry<K,V>(hash, key, value, null);
                         retries = 0;
                     }
                     else if (key.equals(e.key))
                         retries = 0;
                     else
                         e = e.next;
                 }
                 else if (++retries > MAX_SCAN_RETRIES) {
                     lock();
                     break;
                 }
                 else if ((retries & 1) == 0 &&
                          (f = entryForHash(this, hash)) != first) {
                     e = first = f;
                     retries = -1;
                 }
             }
             return node;
         }
 
         private void scanAndLock(Object key, int hash) {
 
             HashEntry<K,V> first = entryForHash(this, hash);
             HashEntry<K,V> e = first;
             int retries = -1;
             while (!tryLock()) {
                 HashEntry<K,V> f;
                 if (retries < 0) {
                     if (e == null || key.equals(e.key))
                         retries = 0;
                     else
                         e = e.next;
                 }
                 else if (++retries > MAX_SCAN_RETRIES) {
                     lock();
                     break;
                 }
                 else if ((retries & 1) == 0 &&
                          (f = entryForHash(this, hash)) != first) {
                     e = first = f;
                     retries = -1;
                 }
             }
         }
 
         final V remove(Object key, int hash, Object value) {
             if (!tryLock())
                 scanAndLock(key, hash);
             V oldValue = null;
             try {
                 HashEntry<K,V>[] tab = table;
                 int index = (tab.length - 1) & hash;
                 HashEntry<K,V> e = entryAt(tab, index);
                 HashEntry<K,V> pred = null;
                 while (e != null) {
                     K k;
                     HashEntry<K,V> next = e.next;
                     if ((k = e.key) == key ||
                         (e.hash == hash && key.equals(k))) {
                         V v = e.value;
                         if (value == null || value == v || value.equals(v)) {
                             if (pred == null)
                                 setEntryAt(tab, index, next);
                             else
                                 pred.setNext(next);
                             ++modCount;
                             --count;
                             oldValue = v;
                         }
                         break;
                     }
                     pred = e;
                     e = next;
                 }
             } finally {
                 unlock();
             }
             return oldValue;
         }
 
         final boolean replace(K key, int hash, V oldValue, V newValue) {
             if (!tryLock())
                 scanAndLock(key, hash);
             boolean replaced = false;
             try {
                 HashEntry<K,V> e;
                 for (e = entryForHash(this, hash); e != null; e = e.next) {
                     K k;
                     if ((k = e.key) == key ||
                         (e.hash == hash && key.equals(k))) {
                         if (oldValue.equals(e.value)) {
                             e.value = newValue;
                             ++modCount;
                             replaced = true;
                         }
                         break;
                     }
                 }
             } finally {
                 unlock();
             }
             return replaced;
         }
 
         final V replace(K key, int hash, V value) {
             if (!tryLock())
                 scanAndLock(key, hash);
             V oldValue = null;
             try {
                 HashEntry<K,V> e;
                 for (e = entryForHash(this, hash); e != null; e = e.next) {
                     K k;
                     if ((k = e.key) == key ||
                         (e.hash == hash && key.equals(k))) {
                         oldValue = e.value;
                         e.value = value;
                         ++modCount;
                         break;
                     }
                 }
             } finally {
                 unlock();
             }
             return oldValue;
         }
 
         final void clear() {
             lock();
             try {
                 HashEntry<K,V>[] tab = table;
                 for (int i = 0; i < tab.length ; i++)
                     setEntryAt(tab, i, null);
                 ++modCount;
                 count = 0;
             } finally {
                 unlock();
             }
         }
     }
 //Get方法
     public V get(Object key) {
         Segment<K,V> s;
         HashEntry<K,V>[] tab;
         int h = hash(key);
         long u = (((h >>> segmentShift) & segmentMask) << SSHIFT) + SBASE;//先获取Segment[] 的下标
         if ((s = (Segment<K,V>)UNSAFE.getObjectVolatile(segments, u)) != null && (tab = s.table) != null) { //用Unsafe的方法获取到Segment对象
             //用Unsafe的方法HashEntry对象，然后遍历链表
             for (HashEntry<K,V> e = (HashEntry<K,V>) UNSAFE.getObjectVolatile(tab, ((long)(((tab.length - 1) & h)) << TSHIFT) + TBASE); e != null; e = e.next) {
                 K k;
                 if ((k = e.key) == key || (e.hash == h && key.equals(k)))
                     return e.value;
             }
         }
         return null;
     }

JDK1.8：

JDK1.8时，ConcurrentHashMap又放弃了分段式加锁的思想，而且也不在用Segment[]数组存值，而是采用在HashMap1.8的基础上，采用Node[] + 链表 + 红黑树 + CAS+ Synchronized 的思想保证线程安全。而且在扩容的时候，不仅要满足链表结构大于8，还要满足数据容量大于64。

 //最大容量
     private static final int MAXIMUM_CAPACITY = 1 << 30;
 //默认容量
     private static final int DEFAULT_CAPACITY = 16;
 //最大数组长度
     static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;
 //默认并发等级（1.7遗留,兼容以前版本）
     private static final int DEFAULT_CONCURRENCY_LEVEL = 16;
 //加载因子（1.7遗留,兼容以前版本）
     private static final float LOAD_FACTOR = 0.75f;
 //转换为红黑树的阈值（道理和HashMap1.8中一样，这里指链表长度）
     static final int TREEIFY_THRESHOLD = 8;
 //红黑树反转成链表的阈值
     static final int UNTREEIFY_THRESHOLD = 6;
 //转换为红黑树的阈值（这里指数组长度）
     static final int MIN_TREEIFY_CAPACITY = 64;
 //扩容转移时的最小数组分组大小
 private static final int MIN_TRANSFER_STRIDE = 16;
 //本类中没提供修改的方法 用来根据n生成位置一个类似时间搓的功能
 private static int RESIZE_STAMP_BITS = 16;
 // 2^15-1，help resize的最大线程数
 private static final int MAX_RESIZERS = (1 << (32 - RESIZE_STAMP_BITS)) - 1;
 // 32-16=16，sizeCtl中记录size大小的偏移量
 private static final int RESIZE_STAMP_SHIFT = 32 - RESIZE_STAMP_BITS;
 //表示正在转移
 static final int MOVED = -1;
 // 表示已转换为红黑树
 static final int TREEBIN = -2;
 // 保留
 static final int RESERVED = -3;
 // 用在计算hash时进行安位与计算消除负hash
 static final int HASH_BITS = 0x7fffffff;
 // 可用处理器数量
 static final int NCPU = Runtime.getRuntime().availableProcessors();
 //构造函数
     public ConcurrentHashMap(int initialCapacity, float loadFactor, int concurrencyLevel) {
         if (!(loadFactor > 0.0f) || initialCapacity < 0 || concurrencyLevel <= 0)//判断参数是否大于等于0
             throw new IllegalArgumentException();
         if (initialCapacity < concurrencyLevel)
             initialCapacity = concurrencyLevel;   // 容量最小为16
         long size = (long)(1.0 + (long)initialCapacity / loadFactor);
         int cap = (size >= (long)MAXIMUM_CAPACITY) ?
             MAXIMUM_CAPACITY : tableSizeFor((int)size);//获取数组容量并保证不大于最大容量
         this.sizeCtl = cap;
     }
 //Put方法
     public V put(K key, V value) {
         return putVal(key, value, false);
     }
 
     final V putVal(K key, V value, boolean onlyIfAbsent) {
         if (key == null || value == null) throw new NullPointerException();//不允许存null
         int hash = spread(key.hashCode());//计算hash值
         int binCount = 0;
         for (Node<K,V>[] tab = table;;) {
             Node<K,V> f; int n, i, fh;
             if (tab == null || (n = tab.length) == 0)
                 tab = initTable();//懒加载，数组为空时初始化
             else if ((f = tabAt(tab, i = (n - 1) & hash)) == null) {//类似Unsafe获取对象的方法获取对象
                 if (casTabAt(tab, i, null,
                              new Node<K,V>(hash, key, value, null)))//CAS，如果获取位置为空，则将数据存入
                     break;
             }
             else if ((fh = f.hash) == MOVED)//首地址处不null 并且Node的hash是-1  表示是ForwardingNode节点正在rehash扩容
                 tab = helpTransfer(tab, f);//帮助扩容的方法
             else {
                 V oldVal = null;
                 synchronized (f) {//加锁
                     if (tabAt(tab, i) == f) {
                         if (fh >= 0) {
                             binCount = 1;
                             for (Node<K,V> e = f;; ++binCount) {
                                 K ek;
                                 if (e.hash == hash &&
                                     ((ek = e.key) == key ||
                                      (ek != null && key.equals(ek)))) {//遍历链表，如果key重复，值替换，老值返回出去
                                     oldVal = e.val;
                                     if (!onlyIfAbsent)
                                         e.val = value;
                                     break;
                                 }
                                 Node<K,V> pred = e;
                                 if ((e = e.next) == null) {
                                     pred.next = new Node<K,V>(hash, key,
                                                               value, null);
                                     break;
                                 }
                             }
                         }
                         else if (f instanceof TreeBin) {//如果为红黑树的对象，调用红黑树的put方法
                             Node<K,V> p;
                             binCount = 2;
                             if ((p = ((TreeBin<K,V>)f).putTreeVal(hash, key,
                                                            value)) != null) {
                                 oldVal = p.val;
                                 if (!onlyIfAbsent)
                                     p.val = value;
                             }
                         }
                     }
                 }
                 if (binCount != 0) {
                     if (binCount >= TREEIFY_THRESHOLD)
                     //如果链表数据超过指定阈值，转换红黑树，并且会再进行一次判断，看数组容量是否大于64，数组大于64后才会转换为红黑树
                         treeifyBin(tab, i);
                     if (oldVal != null)
                         return oldVal;
                     break;
                 }
             }
         }
         addCount(1L, binCount);
         return null;
     }
 //Get方法，基本和1.8HashMap一样，获取数组坐标，然后获取Node对象，并且没有加锁。
     public V get(Object key) {
         Node<K,V>[] tab; Node<K,V> e, p; int n, eh; K ek;
         int h = spread(key.hashCode());
         if ((tab = table) != null && (n = tab.length) > 0 &&
             (e = tabAt(tab, (n - 1) & h)) != null) {
             if ((eh = e.hash) == h) {
                 if ((ek = e.key) == key || (ek != null && key.equals(ek)))
                     return e.val;
             }
             else if (eh < 0)
                 return (p = e.find(h, key)) != null ? p.val : null;
             while ((e = e.next) != null) {
                 if (e.hash == h &&
                     ((ek = e.key) == key || (ek != null && key.equals(ek))))
                     return e.val;
             }
         }
         return null;
     }

注意：分析ConcurrentHashMap源码前，需要先了解一下CAS、Unsafe、Synchronized 、Volatile相关知识。